1. Field of the Invention
The present invention relates to a high-pressure pump with a device for regulating the flow rate for a fuel injection system of an internal combustion engine.
2. Description of the Related Art
As is known, in modern internal combustion engines, the high-pressure pump is designed to send fuel to a common rail having a pre-set accumulation volume of pressurized fuel, for supplying a plurality of injectors associated to the cylinders of the engine. To obtain a proper atomization of the fuel, this must be brought to a very high pressure, in the region of 1600 bar in the conditions of maximum power of the engine. The pressure of the fuel required in the common rail is in general defined by an electronic control unit as a function of the operating conditions, i.e., the running conditions of the engine.
Injection systems are known in which a by-pass solenoid valve, arranged on the delivery pipe of the pump, is controlled by the control unit for draining off directly the excess fuel just pumped into the usual fuel tank, before said fuel enters the common rail.
Since the flow rate of the high-pressure pump in general depends upon the speed of rotation of the engine crankshaft, it must be sized so as to achieve the maximum flow-rate and pressure values required by the various operating conditions of the engine. In certain operating conditions, for example at the maximum speed but with low power of the engine shaft, the flow rate of the pump proves overabundant, and the fuel in excess is simply drained off into the tank. Consequently, these known regulation devices present the drawback of dissipating part of the work of compression of the high-pressure pump in the form of heat.
Variable-flow-rate high-pressure pumps have been proposed to reduce the amount of fuel pumped when the engine functions at low power. In one of these pumps, the intake pipe is provided with a flow-rate regulation device comprising a restriction with a cross section that varies with continuity, which is controlled by the electronic-control unit as a function of the pressure required in the common rail and/or as a function of the operating conditions of the engine.
In particular, the restriction in the intake pipe is supplied with a constant pressure difference ΔP of approximately 5 bar, supplied by an auxiliary pump. By varying with continuity the effective area of passage of the fuel, using a regulation solenoid valve, there is obtained a modulation of the amount taken in by the hydraulically connected pumping elements. The amount of fuel downstream of the regulation solenoid valve, i.e., the amount allowed at intake, is at a very low pressure and yields, at low flow rates, only a small contribution of force for opening the intake valve of the pumping element. Consequently, the usual return spring of the intake valve must be such as to guarantee opening thereof even at a minimum pressure downstream of the restriction. On the one hand, said spring must be calibrated in a very precise way, so that the pump proves relatively costly, and, on the other hand, there is always the risk that the intake valve will be unable to open on account of the negative pressure caused by the pumping element in the corresponding compression chamber, so that the pump does not function correctly and is highly subject to deterioration.